A tap changer is a device fitted to power transformers for regulation of output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer(s). Tap changers cause more failures and outages than any other component of a power transformer. Tap changer failures are categorized as electrical, mechanical or thermal. Many failures begin because of mechanical problems with contacts, transition resistors or insulation breakdowns.
It is important to monitor the condition of a tap changer to potentially avoid failures or outages of the transformer. Historically, to determine a tap changer's condition, a tap changer compartment would be de-energized and physical measurements of components of the tap changer would be taken. Physical observation of the components would also assist in determining the condition of the tap changer.
Some systems have been developed that enable tap changer to be evaluated on-load without affecting its normal operation and requiring de-energizing. These systems use a combination of acoustic emission and vibration techniques (AE/VA). Acoustic Emission assessment is based on the fact that no acoustic activity is expected from inside the tap changer compartment if the tap changer is not being operated and it is in good condition. Vibration techniques include obtaining a signature of one operation of the tap changer and performing a comparison of its characteristics (time, amplitude, energy, etc.) with another signature obtained some time in the future or with another unit having the same operation. When using a combination of both techniques, evaluation of the condition of the tap changer in an off-load state is performed using acoustic emission whereas on-load evaluation is made using the vibration technique.
These systems, however, have drawbacks. For example, the vibration technique may require complex analysis that is costly to perform. Additionally, these systems do not monitor displacement of components within the tap changer. Displacement monitoring provides a good indication of how much wear has occurred to a tap changer component. Furthermore, tap changer compartments contain oil that impedes various types of sensors from obtaining accurate measurements.
These and other drawbacks exist.